Steam stimulation in-situ combustion backflow process



w. M. BLACK ETAL 3,369,604

STEAM STIMULATION INSITU COMBUSTION BACKFLOW PROCESS Filed Oct. 22, 1965FIG.

FIG. 2.

INVENTORS. WILLIAM M. BLACK, BY JOHN H. NICHOLS ATTORNEY.

United States Patent 6) W ABSTRACT F THE DISCLOSURE A method forstimulating subsurface oil-containing formations by steam stimulationand in situ combustion. Steam is first introduced into a subsurfaceformation through an injection-production well to condition the wellbore and formation for the following in situ combustion step. The steamis followed by an oxygen-containing gas which causes spontaneousignition and burning of a portion of the formation oil to raise thetemperature of the formation remote from the Well bore and reduceviscosity of the oil therein. Injection of the oxygen-containing gas isthen halted and the formation oil is pro duced through theinjection-production well.

The present invention concerns stimulation of subsurface oil-containingformations in order to increase production of oil. More particularly,the present invention concerns two known techniques used to stimulateviscous oil wells, steam stimulation and in situ combustion. The samewell is used for the injection of stimulation fluids and the productionof oil.

Steam stimulation is the addition of thermal energy to a subsurfacereservoir by injection of steam, usually of 60 to 90 percent quality,through a producing well for a period, following which the well may beclosed in for a period prior to being returned to productive status.This technique is often referred to as steam soaking, and in thestimulation of viscous oil wells it is widely practiced. It has two maineffects, reduction of oil viscosity and well bore cleanup, includingremoval of skin effect around the well bore. For purposes herein, byskin effect is meant a skin or cylinder of reduced fluid conductivityabout the well bore as described on page 303 of Applied PetroleumReservoir Engineering, by Craft and Hawkins, published by Prentice-Hall,Inc., 1959. In general, steam is injected into the subsurface formationto be stimulated in quantities suflicient to heat up to about feet ofthe formation radially from the well bore. The pressures may range from500 p.s.i.g. to 2500 p.s.i.g. depending upon depth of the formation andthe permeability thereof. Also, in general, steam may be injected in therange of 20,000 pounds/hour, more or less, for seven days to six months.A typical field operation involves 20 to days of steam injection (10 to15 billion B.t.u. injected). This quantity of heat usually produces atwentyto fifty-fold immediate production increase when the well isplaced on production following the steam injection and soak period.However, this stimulation does not last long, and, in addition, thetotal volume of reservoir heated is not large.

In situ combustion refers to a process in which a portion of theunderground oil is burned in place to provide high temperatures and heatand cause a reduction in viscosity of the formation oil adjacent theburned out interval, or which may be caused to enter the burnedinterval. In this process an oxidizing gas, such as air, or a mixture ofair and oxygen or other gases capable of sustaining combustion of theformation hydrocarbons, is introduced into the formation through theproduction well bore. Then, the combustible mixture is ignited in anydesired manner as by electric bore hole heaters or chemical 3,309,504Patented Feb. 20, 1968 catalysts, such as phosphorus, triethylborane,linseed oil, etc., or, most economically, purely by spontaneouscombustion. The oxidizing gas is continuously supplied to the formationto maintain combustion of the subsurface hydrocarbons. When the desiredquantity of heat has been generated by such combustion, injection of theoxidizing gas is discontinued, and burning ceases. In the technique ofin situ combustion stimulation, the well is then returned to productivestatus. The rate of injection of air or other oxidizing gas may rangefrom 50 to 2,000 M c.f./d. for seven days to three months. The rate andtime period to be employed in a particular case will depend uponcharacteristics of the reservoir treated in this manner.

In practice, in situ combustion stimulation yields fiveto ten-foldproduction increases after seven to ninety days injection of air at 2000M c.f./ d. This stimulation effect is usually not as large in terms ofpeak fold (multiple) increases as with the steam soaking technique;however, the stimulation often persists for a much longer time withequal or greater ultimate stimulated oil volumes.

The multiple production increases achieved by either the steam soakingtechnique or in situ combustion technique are larger than theoreticalconsiderations would suggest, unless the existence of a skin effectprior to stimulation is taken into account. The present invention takesinto consideration the fact that steam soak relatively safely removes orreduces the skin effect, while the average in situ combustion operationdoes not (unless ignition techniques are employed which will causeignition inside or within inches of the well bore, in which event, thereis serious hazard of burning up the well casing or liner). Calculationsprovided later herein illustrate this concept.

Experience with in situ combustion stimulation indicates that even anincreased skin effect may result in the average in situ combustionoperation where combustion does not or intentionally is not allowed tobegin at or in the well liner. This stems from creation of a zone orcylinder of weathered or partially oxidized crude oil Within thepro-ignition radius. In addition, laboratory experiments have shown thatspontaneous ignition is very difficult (at least very slow) to achievewhen ambient reservoir temperature is less than 200 F., i.e., theautooxidation reaction is very slow for the first of temperature risefrom, e.g. 100 to 200 F., after which it proceeds quite rapidly toignition temperature of 500 to 700 F. Consequently, steam injection isuseful to raise the temperature of the reservoir to a level (greaterthan 200 P.) where the reaction rate is sufficiently high to quicklyachieve ignition temperatures via autooxidation. Additionally, a periodof steam injection will improve the injectivity profile or uniformity ofentry of a gaseous oxidizing material, such as air, that may be injectedsubsequently.

A primary purpose of the present invention, then, is to producesynergistically, stimulation multiple production increases comparable tosteam soak with total stimulation volumes comparable to or larger thanin situ combustion or steam soak stimulation operations alone.

Briefly, then, the invention comprises a method for stimulating viscousoil producing wells by injecting steam and air in sequence. Moreparticularly, the invention cornprises the steps of introducing steaminto a subsurface hydrocarbon-containing reservoir through a productionwell for a selected period of time sufiicient to remove skin andprecondition the completion interval to receive a following oxidizinggas more uniformly; halting introduction of the steam; then, injectingan oxidizing gas into the subsurface formation through the productionwell, igniting the reservoir oil while continuing injection of theoxidizing gas and burning a portion of the resident reservoirhydrocarbons to raise the temperature of the and in situ combustion for30 days, after which the hydrocarbons are produced. The well bore skinis removed or reduced by the steam and the oil viscosity remote in thereservoir is reduced by the in situ combustion. The completion intervalis preconditioned to receive the oxidizing gas (air) more uniformly bysteam cleaning. Further, the oil in the production pay zone may be morequickly spontaneously ignited for in situ combustion because thetemperature of the oil is raised by the heat transmitted to it by theinjected steam.

A more economical and effective method of treating or stimulatingviscousoil producing wells is achieved by combining in sequence thetechniques of steam soak and underground combustion than either thesteam soak method or cyclic underground combustion method alone afford.

The above object and other objects and advantages of the invention willbe apparent from a more detailed description thereof when taken with thedrawing in which:

FIGS. 1 and 2 are schematic views of a well bore vanclvarious zones ofinterest surrounding it.

Calculations to illustrate theadvantages of combining steam soak and insitu combustion according to the invention are set forth below. Thecalculations demonstrate semiquantitatively the merits of a wellstimulation procedure involving a short steam soak followed byconventional in situ combustion stimulation treatment.

Referring to FIG. 1:

Let

r =Radius of the well liner or perforated casing r Radius of a zone ofrestricted permeability due to a primary depletion plugging mechanism(skin) (asphalteen deposition and fines accumulation, for example) r=Radius of heated zone r =Drainage radius of well.

And from FIG. 6.32 of Craft & Hawkins Ref. PR: 1.87.

(C) PR of well after only conventional in situ stimulation; (skin isnotadequately removed) Let K.,=K and ,t,=0.6/50

The expansion of Craft & Hawkins basic Equation 6.36 on page 295 ofRef.to provide three zones instead of 2 zones, i.e. radial zones (see FIG.2), is as follows:

(D) PR of well after steaming to remove skin and then in situ combustionLet r =O.33 ft.; r doesnt exist; r =24 ft. K K, and g .3 Virgin wen 58fold production Increase =47 fold production increase 10 fold productionincrease The foregoing numerical results are in general agreement withfield experience for steam soak and cyclic in situ combustiontechniques. In one reservoir 27-day steam soaks onlOO ft. sd for 12billion B.t.u. injected produced twentyto fifty-fold productionincreases, which, however, declined to twoto three-fold in a few months.In another reservoir in which an in situ combustion operation wascarried out, fiveto ten-fold increases were obtained which took one totwo years to decline to twoto three-fold.

The calculated numerical results above are controlled largely by theassumedseverity of skin damage rather than the values assumed for the ror radius of heating; consequently, the (r steam) (r burn) is not animportant factor. These values could be considerably different from theassumed ones without substantially affecting the results. Also the ratioof (AP steam/AP burn) is ignored, i.e., assumed equal.

It is also apparent that the skin must exist for the multiple increasesto be on the order shown above. Without skin the denominator in A, B, C,under Resulting Stimulation Ratios heading, would be 1.0 and thecorresponding multiple increases would be 1.9 and 2.3. for steam soakand steam soak plus combustion, respectively.

Having fully described the method, operation, objectives and advantagesof the invention, we claim:

1. A method of well stimulation comprising the steps introducing steaminto a subsurface hydrocarbon-containing reservoir through an injectionproduction Well for a selected period of time sufficient to remove skineffect and pre-condition the completion interval to receive a followingoxidizing gas more uniformly and to raise the temperature of saidreservoir to a reaction temperature sufiicient to ignite spontaneouslysaid hydrocarbons upon introduction of said following oxidizing gas;

halting introduction of steam;

thereafter injecting said oxidizing gas into said subsurface reservoirthrough said production well and igniting the reservoir oil, continuinginjection of said oxidizing gas and burning a portion of the residentreservoir hydrocarbons to raise the temperature of reservoir rock remotefrom the well how, thereby reducing the viscosity of remaining reservoirhydrocarbons;

thereafter halting injection of said oxidizing gas; and

then producing said reservoir hydrocarbons through said production well.

2. A method as recited in claim 1 in which said oxidizing gas is air.

. about seven days to three months at the rate of from 50' to 2000million cubic feet per day.

References Cited UNITED STATES PATENTS 6/ 1958 Walter 16639 X 4/1964Sharp 16611 4/ 1965 Strange 166-11 7/1966 Dietz 16611 FOREIGN PATENTS8/1939 Great Britain.

STEPHEN J. NOVOSAD, Primary Examiner.

